Current balance relay system



Oct. 24, 1944. I LQF. HUNT 2,361,208

CURRENT BALANCE RELAY SYSTEM Filed Aug. 26. 1942 WITNESSES: I INVENTORMm? 1 es m Q BY A'I TORNEY Patented Oct. 24, 1944 CURRENT BALANCE RELAYSYSTEM Lloyd F. Hunt, Glendale, Calif.,-assignor to WestinghouseElectric & Manufacturing Company, East Pittsburgh, Pa., a corporation ofPennsylvania Application August 26, 1942, Serial No. 456,207

12 Claims.

My invention relates to differential relay systems, and it has moreparticular relation to auxiliary apparatus and connections for properlysafeguarding and correlating the operations of a high-speedcurrent-balance relay-system for parallel-line protection, particularlyin a three phase system Where there are a number of highspeedcurrent-balance relays, usually one for each phase, and one for groundor residual-current response.

As pointed out in the Klemmer Patent No. 2,005,144, granted June 18,1935, it is possible, under certain circumstances, for variousdifficulties to be encounteredin the operation of differential relayingsystems for opening both ends of a faulty line-section on a transmissionsystem having two parallel-connected line-sections between two differentstations. These difficulties can arise in several ways.

Thus, the breaker at one end of the faulty linesection may open soonerthan the breaker atthe other 'end, resulting in a momentary heavy rushof reverse-power current in the sound line-section, which may result inerroneously tripping out the sound line-section, either in response tothe high-speed differential protective relaying apparatus, or inresponse to the fault-responsive relaying apparatus which is associatedwith each of the line-sections, regardless of conditions .on th otherline-section.

It is possible, under certain circumstances, for difficulties to arisefrom the fact that one pole of any polyphase breaker will, in general,interrupt its arc in advance of the arcing-contacts in the other phases,resulting in a residual current momentarily appearing on the sound line,with the possibility of an erroneous tripping-response of the sound lineas a result thereof.

Another difficulty which might possibly arise, under extreme conditions,if suitable safeguarding means were not adopted, could occur in thereadjustment-period after a tripping-operation, when certain interlocksmight open at a moment when some one of the tripping-relays is in a po.sition indicating the desirability for tripping out the sound line afterthe faulted one of the parallel line-sections has been switched out, atone or both of its ends.

The above-mentioned difficulties are particularly true of veryhigh-speed relays, particularly when operating under heavy fault-currentcon,- ditions, resulting in a double-frequency pulsating torque oncertain types of relays, tending strongly to vibrate thetripping-contacts between opposite positions, indicating first that onelinesection is faulted, and then that the other linesection is faulted.In pulsating-torque relays of this nature, it has been common, in thepast, to provide each relay with two direct-current holding coils, onefor each position of the relay, these holding-coils being connected inseries with the respective tripping-contacts of the relay, so as to holdthe relay in contact-maintaining position, whenever either one of itstripping-contacts is closed, corresponding to a fault on the one or theother of the parallel-connected line-sections.

When there are a number of such current-balance relays having theirrespective tripping-circuits connected in parallel, one circuit fortripping the breaker on one line, and another circuit for tripping thebreaker on the other'line, care must be exercised to obtain enoughenergy in the holding-coils without interfering with the quick andreliable operation of the tripping-coil of the breaker, and thisrequirement has been particularly hard to meet, in'the' case of a secondbalance-relay pulling in, or responding, after the first relay hasresponded. Thus, on a phase-tophase fault, involving phases A and B, thedifferential or balance-current relay comparing the phase-A current ofthe faulted line with the phase-A current of the sound,parallel-connected line, may respond first, clo'singits tripping-contactfor the faulted line, and simultaneously energizing its holding-coilwhich is connected in series therewith. The phase-B balance-currentrelay may then close its tripping-contact, but its holding-coil wouldthen be connected in parallel with the holding-coil circuit of thephase-A relay, which has a low voltage-drop across it, and thus thephase-B relay under extreme conditions, might conceivably fail toreceive enough energy to keep its phase-B tripping-contacts frombouncing right open again, under the influence of the pulsatingrelay-torque, thus producing a condition which might result,subsequently, in a faulty tripping-operation, if the safeguarding orinterlocking-means, which are provided, should prove inadequate fortheir task.

While some or all of the above-mentioned difficulties and possibilitieshave been known before, and While some or all of them may have been moreor less completely safeguarded against, by one means or another, inprevious apparatus such as that shown in the Klemmer patent, I haveprovided means Wh'ereby certain different safeguarding-steps oroperations may be carried out, which have certain advantages undercertain cases.

Thus, it is an object of my invention to prevent theothercurrent-balance relays from having an opposite direction ofcontact-closing movement, after any one of a bank of current-balancerelays has responded; or to cause the other current--' balance relays torespond in the same direction of contact-closing movement as the firstrelay; or to energize only the holding-coils corresponding to thedesired direction of response, of the entire bank of current-balancerelays, after any one of them has responded; or to energize some or allof the holding-coils from a circuit which is separate from the trippingcircuit; or-to make it impossible for the fault-responsive trippingmeanswhich is associated individually'with the sound line from becomingactuated toward a tripping position during, or immediatelyafter, theclearing of a fault on a faulted parallel-connected line, thus avoidingthe necessity for relying entirely on interlocking trip-circuit contactsin series with the tripping-contacts of relays which would otherwise bepermitted to move to erroneous tripping-conditions, with resultantdanger of tripping out the sound line.

With the foregoing. and other objects in view, my invention. consistsin. the apparatus. parts, combinations, systems, and methods hereinafterdescribed and claimed, and illustrated in the accompanying drawing, thesingle figure of which is a simplified diagrammatic view of circuits andapparatus embodying my invention in an illustrative form.

I have illustrated my invention as being applied to the protection of athree-phase transmission-line comprising two parallel-connectedline-sections I and 2, which are connected, at each end, to a bus 3,through circuit-breakers CBI and CB2,-respectively, only one.-end beingillustrated. The line-phasesare designatedby the letters A, B, and C,

Each circuit breaker, such as the circuitbreaker CBI, is provided with acircuit-interrupting arcing-mechanism 4, two or more auxiliary switches5 and 6, and a tripping-mechanism 'I. The trip-coils for the twobreakers CBI and CB2 are designated TCI and T02, respectively.

The two parallel-connected line-sections I and 2 are provided withcurrent-balance relay-protection by means of four high-speeddifferential relays HDA, HDB, HDC, and 'I-ICG, for phases A, B, and C,and ground or residual-current, re-

spectively. Each of the high-speed differential or current-balancerelays, such as the relay HDA, is provided with twooppositely actingalternatingcurrent operating-coils II and I2,which are energized fromthe corresponding phase or groundcurrents from two banks ofcurrent-transformers CTI and GT2, associated-with the respective lines Iand 2. Each current-balancerelay is also provided with a centrallybiased, movablearm I3, which is moved, in. one direction or the other,to close either a tripping-contact 2|, in case of a fault onv the-lineI, or a tripping-contact 22, in case of a fault on the line 2.- Eachbalancecurrent relay is also provided with two directcurrent holdingcoils HCI and I-IC2, for holding the relay" in. its actuated orcontact-making position, for the contacts 2| and 22, respectively,corresponding to thefaults on the lines I and 2, respectively.

In addition to the differential, or current-balance, relays HDA, HDB,HDC, and HCG, each of the two line-sections has an individualfault-responsive relaying means, RI and R2, respectively, associatedtherewith, for responding. to predetermined fault-conditions on eitherline-section alone, regardless of conditions on the other linesection. Ihave shown the current-coils 30 of the respective fault-responsiverelays RI and R2 energized from the respective line-current transformersCTI and GT2, in series with the respective operating-coils II and I2 ofthe currentbalance relays. Each of the individual faultresponsive relaysRI and R2 is provided with its own tripping-contact, or a plurality ofcontacts, as indicated illustratively at 3| and 32, respectively, fortripping the respective line circuit-interrupters CBI and CB2.

I have also shown banks of double-throw switches 33 whereby theoperating coils II and I2 of the various current-difierential relaysHDA, HDB, HDC, and HDG may be switched out 01' service, leaving each ofthe line-sections I and 2 to be protected only by its individualfault-responsive relaying-means RI and R2, respectively. Thecutout-switches 33 are shown in the position with the diiferential orcurrent-balance relaysconnected in service.

In accordance with my present invention, I provide a series of auxiliaryrelays AXI and AI for the line I, and AX2 and A2 for the line 2. Theoperating coils of. the relays AXI and AI are illustrated as beingconnected in series with each other, between the positivebattery-terminal and an intermediate relaying bus or connection 35, andthence, through any one of the four-current-balance line I contacts H,to a second relaying bus or connection 36, from which thecircuit-connection is completed through the auxiliary switches 5 of bothof the line circuit-breakers CBI and CB2, the circuit being completedatthe negative battery-terminal at the circult-breaker CB2. Theseauxiliary interlocking breaker-switches 5 are closed when the breaker isclosed, and open when the main breaker-contacts are approximately open.In like manner, the operating coils of the auxiliary relays AX2 andA2'are also illustrated as being connectedin series with each other,between the positive battery-terminal through the interlockingbreaker-switches 5, as already described.

The auxiliary switches AI and A2 are thus energized when any one of thedifferential or current-balance relays indicates a fault on line I orline 2, as the case may be. Each of these auxiliary relays AI and A2 isshown as being provided with three make-contacts. The makecontacts 38and 39 of relays AI and A2, respectively, are utilized to bypass thecurrent-balance contacts 2| or 22, as the case may be, which caused theinitial operationor response of the auxiliary relays AI and A2, thusholding in, whichever relay is operated, andmaintaining its circuituntil the corresponding circuit-breaker CBI or CB2, respectively, hasopened its contacts far enough to break the relaying-circuit at theauxiliary breaker-switch 5.

The respective auxiliary relays AI and A2 also have trip-contacts 4| and42 for directly energizing the trip-coils TCI and T02, respectively, inparallel with the fault-responsive relay-contacts 31- and 32,respectively. These trip-coil circuits are completed through theauxiliary breaker-switches 6 of their respective breakers, so that eachtrip-coil circuit is finally interrupted when. its owncircuit-interrupting arcing-mechanism is about 50% open, or in any othercondition of opening according to the adjustment of the breaker-switch6.

The auxiliary relays AI and A2 also have contacts SI. and 52 which areutilized to complete a separate energizing-circuit for the holding-coilsHCI and HC2, as the case may be, for the respective differential relaysHDA, I-IDB, HDC, and HDG, the four line-l holding coils HCI beingillustrated as being connected in series with each other, and energizedfrom the relay-contact from the positive battery-terminal to thenegative battery-terminal while the four line-2 holding-coils H02 aresimilarly illustrated as being serially connected in circuit with therelay-contact 52, although it is to be understood that a parallelconnection of the respective groups of four holding-coils HCI or H02might have been utilized, instead of the series connection; it beingpreferable, however, that these holding-coils I-ICI and HC2 shall beenergized in circuits which are independent of the trip-coil circuitswhich are controlled by the relay-con tacts 4| and, 42, respectively.

It will be noted that the other auxiliary relays, AXI and AX2,respectively, are energized, respectively, when the respective lines Iand 2 are faulted. When the auxiliary relay AXI picks up, in response toa differential-current relay-operation indicating a fault on the line I,it closes a bank of contacts 54 which are utilized to shortcircuit thefour terminals of the line-current transformer GT2 which is associatedwith the other line, that is, with the sound line 2. In like manner,when the auxiliary relay AXZ picks up,

in response to a differential-relay indication of .a fault on theline-section 2, it closes a bank of relay-contacts 55 whichshort-circuit the four terminals of the line-current transformer CTIwhich is associated with the other, o sound, linesection I. In eithercase, the short-circuiting of the current-transformer terminalsassociated with the sound line-section effectively prevents afault-indicating response, to any currents in the sound line-section, byany one of the four differential relays I-IDA, HDB, HDC, or HDG, or bythe corresponding fault-responsive relay RI or R2, as the case may be.

In this manner, I prevent any possibility of a faulty relaying-operationduring the progress of, or immediately after, the clearing of a fault oneithe one of the parallel-connected lines I and 2, as indicated by theinitial response of any one of the. four current-balance relays HDA,HDB, HDC, or I-IDG. In fact, my short-circuiting of thecurrent-transformer terminals of the sound line-section, in the case ofthe four current-balance relays HDA, HDB, HDC and HDG, not only preventsany one of the relay-contacts l3 from swinging to the sound-line side ofits movement,

but it actually causes all of the other currentbalance relays to followthe lead of the first current-balance relay to respond, and to move overto a contact-making position on the faulted-line side of the relay; thisresult being produced by the energization of the differential-currentrelays with current from only the faulted line-section, as compared topractically no current at all from the sound line-section.

At the termination of the tripping operation, or upon the opening of theauxiliary breakerswitch 5 of the tripped circuit breaker CBI or CB2, asthe case may be, the auxiliary relay AX! or AX2, whichever one wasenergized, will become deenergized. thus removing the short-circuit fromthe terminals of the line-current transformer of the sound line, thusmaking sure that the protective operation is completed before making itpossible for any relay to respond to the current flowing in the soundline-section 2.

From the foregoing explanation of my apparatus and of its operation, itwill be apparent that I have provided a novel means for safeguarding theoperation of a current-balance differential protective-relay system forparallel lines; and while I have illustrated my invention in a singleform of embodiment, which I at present prefer, I wish it to beunderstood that my invention is by no means limited to this particularform of embodiment, particularly in the broader aspects of my invention.I desire, therefore, that the appended claims shall be accorded thebroadest construction consistent with their language and the prior art.

I claim as my invention:

1. Protective relaying-means for a polyphase transmission-line; saidline comprising at least two line-sections capable of being operated inparallel, and a line circuit-interrupter for each line-section; saidprotective relaying-means comprising a plurality of current-balancerelayingmeans, each having a contact for each of the parallel-connectedline-sections for selectively responding to whichever line-section isfaulted, line-current-energized means for differentially energizing eachof the plurality of current-balance relaying-means from both of theparallelconnected line-sections, the energizing-connections beingdifferent for each of the plurality of current-balance relaying-means,and means responsive to a closure of either of the contacts of any oneof the plurality of current-balance relayinvmeans for initiating anopening-operation of the appropriate line circuit-interrupter, and forpreventing an opposite direction of contactclosing movement of any ofthe other currentbalan e relaying-means.

2. Protective relaying-means for a polyphase transmission-line; saidline comprising at least two line-sections capable of being operated inparallel, and a line circuit-interrupter for each line-section; saidprotective relaying-means comprising a plurality of current-balancerelayingmeans, each having a contact for each of the parallel-connectedline-sections for selectively responding to whichever line-section isfaulted,

' line-current-energized means for differentially energizing each of theplurality of current-balance relaying-means from both of theparallelconnected line-sections, the energizing-connections beingdifferent for each of the plurality of current-balance relaying-means,and means responsive to a closure of either of the contacts of any oneof the plurality of current-balance relaying-means for initiating anopening-operation of the appropriate line circuit-interrupter, and forcausing the same direction of contact-closing movement of the othercurrent-balance relayingmeans.

3. Protective relaying-means for a polyphase transmission-line; saidline comprising at least two line-sections capable of being operated inparallel, and a line circuit-interrupter for each line-section; saidprotective relaying-means comprising a plurality of current-balancerelayingmeans, each having a contact for each of the parallel-connectedline-sections for selectively responding to whichever line-section isfaulted, line-current-energized means for differentially energizing eachof the plurality of current-balance relaying-means from both of theparallelconnected line-sections, the energizing-connections beingdifferent for each of the plurality of current-balance relaying-means,and means responsive to a closure of either of the contacts of any oneof the plurality of current-balance relaying-means for initiating anopening-operation of the appropriate line circuit-interrupter, and fordeenergizing theline-current energized energizing-means which suppliesenergy to'the plurality of current-balance relaying-means from the otherparallel-connected line-section.

4. Protective relaying-means for a polyphase transmission-line; saidline comprising at least two line-sections capable of being operated inparallel, and a line circuit-interrupter for each line-section; saidprotective relaying-means comprising a plurality of current-balancerelayingmeans, each having a contact for each of the parallel-connectedline-sections for selectively responding to whichever line-section isfaulted, line-current-energized means for differentially energizing eachof the plurality of current-balance relaying-means from both of theparallelconnected line-sections, the energizing-connections beingdiiferent for each of the plurality of current-balance relaying-means,electrically operated holding-means associated with each of the contactsof each of the plurality of current-balance relaying-means for holdingits contact closed once it is closed, and means responsive to a closureof either of the contacts of any one of the plurality of current-balancerelayingmeans for establishing a relaying-circuit for in itiating anopening-operation of the appropriate line circuit-interrupter, and forestablishing one or more other relaying-circuits for energizing at leastone of the holding-means of all of the plurality of current-balancerelaying-means.

5. Protective relaying-means for a polyphase transmission-line; saidline comprising at least two line-sections capable of being operated inparallel, and a line circuit-interrupter for each line-section; saidprotective relaying-means comprising a plurality of current-balancerelayingmeans, each having a contact for each of the parallel-connectedline-sections for selectively responding to whichever line-section isfaulted, line-current-energized means for differentially energizing eachof the plurality of current-balance relaying-means from both of theparallelconnected line-sections, the energizing-connections beingdifferent for each of the plurality of current-balance relaying-means,operated holding-means associated with each of the contacts of each ofthe plurality of currentbalance relaying-means for holding its contactclosed once itis closed, and means responsive to a closure of either ofthe contacts of any one of the plurality of current-balancerelayingmeans for initiating an opening-operation of the appropriateline circuit-interrupter, and for selectively energizing only thoseholding-means associated with the corresponding contacts for that linecircuit-interrupter in each of the plurality of current-balancerelaying-means.

6. Protective relaying-means for a polyphase transmission-line; saidline comprising at least two line-sections capable of being operated inparallel, and a line circuit-interrupter for each line-section; saidprotective relaying-means comprising a plurality of current-balancerelayingmeans, each having a contact for each of the parallel-connectedline-sections for selectively responding to whichever line-section isfaulted, line-current-energized means for difierentially energizing eachof the plurality of current-balance relaying-means from both of theparallel-connected line-sections, the energizing-connections beingdifferent for each of the plurality of current-balance relaying-means,electrically operated holding-means associated with each of the contactselectrically of each of the plurality of current-balance relaying-meansfor holding its contact closed once it is closed, and means responsiveto a closure of either of the contacts of any one of the plurality ofcurrent-balance relaying-means for initiating an opening-operation ofthe appropriate line circuit-interrupter, for selectively energizingonly those holding-means associated with the corresponding contacts forthat line circuit-interrupter in each of the plurality ofcurrent-balance relaying-means, and for deenergizing theline-currentenergized energizing-means which supplies energy to theplurality of current-balance relayingmeans from the otherparallel-connected linesection.

'7. Protective relaying-means for a polyphase transmission-line; saidline comprising at least two line-sections capable of being operated inparallel, and a line circuit-interrupter for each line-section; saidprotective relaying-means comprising a plurality of current-balancerelayingmeans, each having a contact for each of the parallel-connectedline-sections for selectively responding to whichever line-section isfaulted, fault-responsive relayingrneans having contacts responsive topredetermined fault-conditions on either line-section alone, regardlessof conditions on the otherline-section, line-energized means fordiiferentially energizing each of the plurality of current-balancerelaying-means from the linecurrents of both of the line-sections duringparallel-connected line-operation, and for energizing saidfault-responsive relaying means responsively to predetermined electricalquantities including the line-currents of their respective linesections,the energizing-connections being dif ferent for each of the plurality ofcurrent-balance relaying-means, means responsive to a closure of thecontact of either of said fault-responsive relaying-means for initiatingan opening-operation of its line circuit-interrupter, and meansresponsive to a closure of either of the contacts of any one of theplurality of current-balance relaying-means for initiating anopening-operation of the appropriate line circuit-interrupter, and forpreventing either an opposite direction of contact-closing movement ofany of the other current-balance relaying-means, or a contactclosingresponse of the fault-responsive relaying means associated with theother line-section.

8. Protective relaying-means for a polyphase transmission-line; saidline comprising at least two line-sections capable of being operated inparallel, and a line circuit-interrupter for each line-section; saidprotective relaying-means com prising a plurality of current-balancerelayingmeans, each having a contact for each of the parallel-connectedline-sections for selectively responding to whichever line-section isfaulted, fault-responsive relaying-means having contacts responsive topredetermined fault-conditions on either line-section alone, regardlessof conditions on the other line-section, line-energized means fordifferentially energizing each of the plurality of current-balancerelaying-means from the linecurrents of both of the line-sections duringparallel-connected line-operation, and for energizing saidfault-responsive relaying means responsively to predetermined electricalquantities including the line-currents of their respective linesections,the energizing-connections being different for each of the plurality ofcurrent-balance relaying-means, means responsive to a closure of thecontact of either of said fault-responsive relaying-means for initiatingan opening-operation of its line circuit-interrupter, and meansresponsive to a closure of either of the contacts of any one of theplurality of current balance relayingmeans for initiating anopening-operation of the appropriate line circuit-interrupter, and fordeenergizing the line-current-energized energizingmeans which suppliesenergy to the plurality of current-balance relaying-means and to thefaultresponsive relaying-means for the other parallelconnectedline-section.

9. Protective relaying-means for a transmission-line; said linecomprising at least two linesections capable of being operated inparallel, and a line circuit-interrupter for each linesection; saidprotective relaying-means comprising a current-balance relaying-meanshaving a contact for each of the parallel-connected line-sections forselectively responding to whichever line-section is faulted,fault-responsive relaying-means having contacts responsive topredetermined faultconditions on either line-section alone, regardlessof conditions on the other line-section, lineenergized means fordifferentially energizing the current-balance relaying-means from thelinecurrents of both of the line-sections during parallel-connectedline-operation, and for energizing said fault-responsive relaying meansresponsively to predetermined electrical quantities including theline-currents of their respective line-sections, means responsive to aclosure of the contact of either of said fault-responsive relaying-meansfor initiating an opening-operation of its line circuit-interrupter, andmeans responsive to a closure of either of the contacts of thecurrent-balance relaying-means for initiating an openingoperation of theappropriate line circuit-interrupter, and for preventing either areversal of the contact-movement of the current-balance relaying-means,or a contact-closing response of the fault-responsive relaying-meansassociated with the other line-section.

10. Protective relaying-means for a transmission-line; said linecomprising at least two linesections capable of being operated inparallel, and a line circuit-interrupter for each line-section; saidprotective relaying-means comprising a current-balance relaying-meanshaving a contact for each of the parallel-connected line-sections forselectively responding to Whichever linesection is faulted,fault-responsive relayingmeans having contacts responsive topredetermined fault-conditions on either line-section alone, regardlessof conditions on the other linesection, line-energized means fordifferentially energizing the current-balance relaying-means from theline-currents of both of the line-sections during parallel-connectedline-operation, and for energizing said fault-responsive relaying meansresponsively to predetermined electrical quantities including theline-currents of their respective line-sections, means responsive to aclosure of the contact of either of said fault-responsive relaying-meansfor initiating an opening-operation of its line circuit-interrupter, andmeans responsive to a closure of either of the contacts of thecurrent-balance relaying-means for initiating an opening-operation ofthe appropriate line circult-interrupter, and for preventing acontactclosing response of the faultresponsive relaying-means associatedwith the other line-section.

11. Protective relaying-means for a transmission-line; said linecomprising at least two linesections capable of being operated inparallel, and a line circuit-interrupter for each line-section; saidprotective relaying-means comprising a corrent-balance relaying-meanshaving a contact for each of the parallel-connected line-sections forselectively responding to whichever line-section is faulted,fault-responsive relaying-means having contacts responsive topredetermined faultconditions on either line-section alone, regardlessof conditions on the other 1inesection,-lineenergized means fordifierentially energizing the current-balance relaying-means from thelinecurrents of both of the line-sections during parallel-connectedline-operation, and for energizing said fault-responsive relaying meansresponsively to predetermined electrical quantities including theline-currents of their respective line-sections, means responsive to aclosure of the contact of either of said fault-responsive relaying-meansfor initiating an opening-operation of its line circuitinterrupter, andmeans responsive to a closure of either of the contacts of thecurrent-balance relaying-means for initiating an opening-operation ofthe appropriate line circuit-interrupter, and for de-energizing theline-current-energized energizing-means which supplies energy to thecurrent-balance relaying-means and to the faultresponsive relaying-meansfrom the other parallel-connected line section.

12. Protective relaying-means for a transmission-line; said linecomprising at least two linesections capable of being operated inparallel,

' and a line circuit-interrupter for each line-secergizing thecurrent-balance relaying-means from the line-currents of both of theline-sections during parallel-connected line-operation, and forenergizing said fault-responsive relaying means responsively topredetermined electrical quantities including the line-currents of theirrespective line-sections, means responsive to a closure of the contactof either of said fault-responsive relaying-means for initiating anopening-operation of its line circuit-interrupter, and means responsiveto a closure of either of the contacts of the current-balancerelaying-means for initiating an opening-operation of the approriateline circuit-interrupter, and for de-energizing theline-current-energized energizing-means which supplies energy to thefault-responsive relayingmeans from the other parallel-connectedlinesection.

LLOYD F. HUNT.

